Wrapper instances question

[patrick avery]

This one from Inquisition (SCJP 5 quiz by Mark DeChamps) confuses me:

    
public static void main(String... args)
    {       
        Integer x=5;
        Integer y=x;
        y++;
        System.out.println(x);
    }

The answer is 5 but my incorrect reasoning led me to say 6...
It looks like Integer y=x; creates a 2nd object, whereas I interpreted it as assigning y to the same object that x points to.

I.e. in the code below why does Integer y=x; create a new object while AClass m2 = m1; just points m2 to the same object as m1?

    
public static void main(String... args)
    {       
class AClass { int x = 1; }
AClass m1 = new AClass();  
AClass m2 = m1;            // m2 + m1 point to same object?
m2.x++; 
System.out.println("m1.x " + m1.x);// 2
System.out.println("m2.x " + m2.x);// 2
//-----------------------
Integer x=5;
        Integer y=x;              // x + y point to different objects?
y++;
System.out.println("Integer x " + x);  // 5
System.out.println("Integer y " + y);  // 6
    }

[Henry Wong]

The answer is 5 but my incorrect reasoning led me to say 6...
It looks like Integer y=x; creates a 2nd object, whereas I interpreted it as assigning y to the same object that x points to.

Your reasoning for this line is actually correct. When you assign a reference to another reference, it doesn't create a new object.

It's the next line that is the issue. Integer objects are immutable, so when you do y++ on an Integer object, the value must be unboxed, incremented, and reboxed. It is this boxing that changes the reference to another Integer object.

Henry
[ December 09, 2008: Message edited by: Henry Wong ]

[patrick avery]

Thanks, Henry!! That makes perfect sense to me now.
[ December 09, 2008: Message edited by: patrick avery ]